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ODUCTION

THE AA5xxx (Al-Mg) series aluminum alloys were used extensively for applications in automotive structures[1–5] due to their high work hardening rate and good formability.[6] These alloys are presently made from primary aluminum and recovered process scrap. In the longer term, the increased use of postconsumer recycled scrap[8–10] in alloy formulation will inevitably result in higher levels of contamination by impurities such as copper, iron, and silicon, which could have an adverse effect on formability and other properties. These impurities react with other alloying elements, especially magnesium and manganese, to form intermetallic particles (e.g., Al6(FeMn), Alm(FeMn), Al3(FeMn), MgCuAl2, and Mg2Si) during ingot solidification,[11–15] and the type, size, and distribution of these secondphase particles depend on the local solidification conditions.[15–17] These phases affect the rolling process and influence the final sheet properties.[6,7,18] The melt conditioning by advanced shearing technology (MCAST) process can be used to treat melts of aluminum and magnesium alloys under both semiS. KUMAR, Research Fellow, N. HARI BABU, Lecturer, and G.M. SCAMANS and Z. FAN, Professors, are with the EPSRC Centre—LiME, BCAST, Brunel University, Uxbridge, Middlesex, UB8 3PH, United Kingdom. Contact e-mail: [email protected] Manuscript submitted October 15, 2010. Article published online May 19, 2011 METALLURGICAL AND MATERIALS TRANSACTIONS A

solid[19,20] and fully liquid conditions[21–25] and can produce refined cast microstructures with significantly reduced segregation. In the present study, we examined the use of high shear melt conditioning on the tolerance of an AA5754 automotive sheet alloy to the addition of 0.6 wt pct Fe, 0.8 wt pct Si, and 0.5 wt pct Cu. These levels were based on formulation of the AA5754 alloy from a high level of recycled old rolled aluminum sheet as the postconsumer scrap source.

II.

EXPERIMENTAL DETAILS

The composition of the primary metal AA5754 alloy used as the base alloy in this study is given in Table I. The alloy was melted in a clay graphite crucible at a temperature of 1028 K (755 C), and controlled additions of 0.6 wt pct Fe, 0.8 wt pct Si, and 0.5 wt pct Cu were made to the melt as Al-46 wt pct Fe, Al-50 wt pct Si master alloys and as pure copper. The composition of the modified alloy is also shown in Table I. An addition of 0.1 wt pct of Al-5Ti-1B grain refiner was made to the liquid metal 10 minutes before casting. Ingots were then cast with and without high shear melt conditioning. For the conventional casting process, the liquid metal was poured at 993 K (720 C) into a preheated (623 K (350 C)) cast-iron book mold 190 9 166 9 34 mm in size. For casting after intensive shearing, the liquid metal at 953 K (680 C) was poured into the melt VOLUME 42A, OCTOBER 2011—3141

conditioning unit and the melt was then intensively sheared by twin screws rotating at 500 rpm for 60 seconds at 923 K (650 C). The sheared melt was then cast into the preheated book mold. It is important to n